The invention is directed to water dispersible or water soluble polyesters that may be used as binders in coating compositions and the like. In particular, the invention improves the stability of polyesters in aqueous media.
It is known to render polyesters dispersible in water by reacting hydroxyl functionality on a polyester with a polycarboxylic acid or an anhydride, thereby producing half-esters that provide carboxyl sites on the polyester. These carboxyl sites are then neutralized with a base such an amine, hydroxide, or carbonate to render the polyester compatible with water. Such an approach is dlsclosed in U.S. Pat. No. 4,576,990 (Mazaki et al.).
A known difficulty with such waterborne polyesters is that the presence of the base, which is necessary for solubilization, exposes the polyester to saponification reactions, whereby the half-esters may be cleaved from the polyester. As the saponification progresses over a period of time, a polyester that was initially stable in an aqueous medium may gradually lose its stability, and the product may thereby become unsuitable for its intended purpose. Therefore, the prior art has attempted to find ways to stabilize waterborne polyesters against saponification.
Blum, Hohlein, and Meixner, in an article titled "Water Based Coatings with Excellent Saponification Stability," Advanced Organic Coating Science Technology Series, Vol. 11, pages 45-56, hypothesized that saponification of half-ester groups was internally catalyzed by the proximity of the carboxyl group, an effect termed the "anchimeric effect." The authors report several attempts to prevent this anchimeric effect in polyesters, one of which involved the use of substituted succinic acids as the acidifying agent. Although a favorable suppression of the anchimeric effect was found by using substituted succinic acids, it was concluded that "the saponification has also not been improved to such an extent that storage-stable lacquers in aqueous form could be manufactured on an acceptable basis." It may be noted that the initial acid number of the polyesters acidified with substituted succinic acid was 20.7 or 20.8 milligrams KOH per gram of resin.
Instead of acidifying the polyester, another approach taken in the prior art to make waterborne polyester coatings is to employ surfactants that permit the polyester to be dispersed in water. Such an approach is generally less desirable because the presence of the surfactant in the cured coating tends to reduce the water resistance of the coating.
It would be highly desirable to provide a coating that includes polyester as the primary binder resin, that does not require substantial amounts of surfactant, and which is resistant to saponification.